This invention generally relates to imaging systems. In particular, the invention relates to the storage of digital images in memory, whether that memory is incorporated in or connected to the imaging system. Although the field of the invention is wide, encompassing digital imaging systems, the preferred embodiment of the invention will be disclosed with reference to an ultrasound imaging system used in medical diagnostics, with the understanding that the invention has application in other types of ultrasound imagers as well as digital imagers other than ultrasound imagers.
Conventional ultrasound imagers create two-dimensional images of biological tissue by scanning a focused ultrasound beam in a scan plane and for each transmitted beam, detecting the ultrasound wave energy returned along a respective scan line in the scan plane. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point, and then receiving the reflected energy over time. A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel on the display screen is based on the intensity of the echo returned from the biological tissue being scanned. The outputs of receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the scanned object. These pixel intensity values are log-compressed, scan-converted and then displayed as a B-mode image of the anatomy which was scanned.
If the ultrasound probe is swept over an area of body, a succession of image frames (corresponding to spaced slices intersecting the body being examined) can be displayed on the monitor. In one type of ultrasound imaging system, a long sequence of the most recent images are stored and continuously updated automatically in a cine memory on a first-in, first-out basis. The cine memory is like a circular image buffer that runs in the background, capturing image data that is displayed in real time to the user. The cine memory acts as a buffer for transfer of images to digital archival devices via the host computer. When the user freezes the system (by operation of an appropriate device on an operator interface), the user has the capability to view image data previously captured in cine memory. The image loop stored in cine memory can be reviewed on the display monitor via trackball control incorporated in the operator interface, and a section of the image loop can be selected for hard disk storage. Any acquired or projected image can be stored internally on the system hard disk or on a magneto-optical disk (MOD) inserted in a disk drive.
In addition to storing images internally, modern imaging systems need to be able to transfer images to various types of remote devices, such as storage devices, via a communications network. To successfully transfer images, the relevant networking features of the imager must be compatible with the networking features of the destination remote device. In particular, the imager must place the data to be transferred in a format which can be handled by the destination remote device. An attempt to accomplish the foregoing is the adoption of the DICOM (Digital Imaging and Communications in Medicine) standards, which specify the conformance requirements for the relevant networking features. The DICOM standards are intended for use in communicating medical digital images among printers, workstations, acquisition modules (such as an ultrasound imaging system) and file servers. The acquisition module is programmed to transfer data in a format which complies with the DICOM standards, while the receiving device is programmed to receive data which has been formatted in compliance with those same DICOM standards.
The DICOM system is designed to facilitate the communication of digital images of different types, e.g., X-ray, computerized tomography, magnetic resonance and ultrasound imaging. All DICOM activities are handled in a queued manner by application software running on a host computer incorporated in the imager. In one type of ultrasound imager, the user can select any image in cine memory to be sent in DICOM format via a LAN to a remote device having DICOM capability. The host computer of the ultrasound imaging system is programmed with DICOM system software which facilitates transmission of image frames from the cine memory to the remote DICOM device via the host computer hard disk and the LAN.
In the conventional ultrasound imager, images can be sent to a storage device in either an automatic or a manual mode, depending on the user configuration. When the automatic mode is configured, console keys are used to capture the image and to store it on the hard disk. The request is queued to a DICOM queue manager (preferably implemented in software), which requests an association with the destination remote storage device. After the association with the remote storage device has been opened, the queue manager xe2x80x9cpushesxe2x80x9d the image to the remote storage device without user intervention. The transfer is done in the background while scanning or other operator activities continue. In the manual mode, the captured images are archived on the hard disk or on a MOD during the exam(s). Upon completion of the exam(s) the images are tagged using an archive menu and queued to any of the network devices that have been configured on the imager. The images are sent sequentially in the background while scanning or other operator activities proceed.
One of the current problems in the medical industry is the large amount of digital image data which needs to be stored, requiring vast memory capacity. For example, all of the images produced by an ultrasound imaging machine are of static sizes (about 385 kilobytes for black/white image frames and about 1 megabyte for color images. As the number of digital images being saved increases, so does the amount of hard disk space which is required. Because increases in the hard disk space capacity result in corresponding increases in operating costs, hospitals and clinic are seeking ways to decrease the amount of hard disk space used.
One solution to the foregoing problem is data compression. Data compression involves techniques for storing data in a format that requires less space than usual. Compressing data is the same as packing data. There are a variety of data compression techniques.
Lossless compression refers to data compression techniques in which no data is lost. For most types of data, lossless compression techniques can reduce the space needed by only about 50%, i.e., lossless compression techniques achieve a compression ratio at best of about 2:1. For greater compression, one must use a lossy compression technique.
Lossy compression refers to data compression techniques in which some amount of data is lost. Lossy compression technologies attempt to eliminate redundant or unnecessary information. Only certain types of data, e.g., graphics, audio, and video, can tolerate lossy compression. One known lossy data compression technique is JPEG, which stands for Joint Photographic Experts Group. JPEG is a lossy compression technique for color images. Although it can reduce files sizes to about 5% of their normal size, i.e., achieving a compression ratio of up to 20:1, some of the original data is lost in the compression. The resulting image degradation can be detrimental in the context of medical diagnostic imaging.
However, it is possible to compress images, using lossy techniques, with a compression ratio of up to 20:1, while still maintaining diagnostic quality. It should be noted that the compression ratio would vary from one image to the next in terms of maintaining diagnostic quality. For example, an image with primarily black on it will support a higher compression ratio than an image that had a more complex image with multiple colors. Now, it is clear that not all images will have the same compression ratio. As used herein, the term xe2x80x9cdiagnostic qualityxe2x80x9d means that the images have sufficient detail to enable them to be reliably used by a physician in making a diagnosis of the subject""s medical condition.
Studies have shown that lossy compression, up to a certain compression ratio, provides images which are acceptable for use in clinical diagnosis. However, the acceptable compression ratio will vary on a per image basis. This being the case, it would be very difficult for a picture archiving and communications system (PACS) or other receiving device to perform lossy compression automatically, since the receiving device will receive many images from many vendors, all requiring different compression ratios of acceptance. Moreover, the compression ratio will vary from one image to the next in terms of maintaining diagnostic quality. For example, an image with primarily black in it will support a higher compression ratio than an image that has multiple colors. Thus, not all images require the same compression ratio to maintain an acceptable level of diagnostic quality.
Thus there is a need for a method and an apparatus for providing lossy compression of digital images on a per image basis.
The present invention is a method and an apparatus for providing lossy compression of digital images on a per image basis. In accordance with the preferred embodiments of the invention, the system user is able to manually vary the degree or level of compression applied to a frozen image frame, view each resulting compressed image on the display monitor, and determine the highest compression level which still provides an image having sufficient diagnostic quality in the region of interest.
The concept of the invention can be applied to any imaging device which stores images in internal memory or sends images for storage to external memory. In particular, the invention has application in medical diagnostic imaging devices, such as ultrasound scanners, computer-aided tomographic scanners, magnetic resonance imagers, and so forth. In accordance with the preferred embodiments of the invention, the image data in an image frame undergoes lossless compression prior to storage in memory, using a level of compression or a compression ratio which is selected by the system user.
In accordance with the preferred embodiments of the invention, an operator input device is provided on the imaging system that allows the user to manually vary the level of compression or the compression ratio while viewing each compressed image. The level of compression or the compression ratio could be set by turning a dial, by manipulating keys on a keyboard, by translating a calibrated sliding mechanism, by interacting with virtual, i.e., soft, keys of a graphical user interface, or any other suitable operator input device.
The method in accordance with one preferred embodiment of the invention comprises the following steps. During an examination performed on an ultrasound imaging system, the system user will scan a patient by moving a hand-held ultrasound probe over the surface of a part of the patient""s anatomy. When the user has found an image of interest, the user will then operate an input device which actuates a freeze image function. In response to actuation of the freeze image function, the image is frozen on the display monitor of the imaging system. Typically, the frame of image data corresponding to the frozen image is stored in a cine memory (along with a sequence of the preceding image frames).
After the image of interest has been frozen, the user can begin to compress the image data for that image by operating the variable compression input device. As the level of compression is adjusted, a new image having the adjusted compression level is automatically displayed on the screen.
In accordance with the preferred embodiment of the invention, the frame of image data corresponding to the original frozen image is always maintained in memory in uncompressed form. This is important, as the compression is lossy, so the original image must be maintained to allow the user to decrement the level of compression or the compression ratio. Each newly compressed image is derived by applying a respective level of compression to the original frame of uncompressed image data. Only the most recently compressed image is also stored temporarily in the cine memory and then displayed, the next most recent compressed image being overwritten in cine memory at that time.
As the scanner receives signals from the user input device to adjust the compression level, the newly adjusted image frame is displayed on the screen in real-time. The user can then stop adjusting the compression at the point where he/she has determined the image to still have an acceptable clinically diagnostic quality. When the system user decides that the compressed image being viewed is acceptable for storage, the user will then operate an input device which actuates a save image function. In response to actuation of the save image function, the compressed version of the frozen image is captured from cine memory and stored in the destination storage device, whether the latter is the imaging system hard disk, a storage device connected to the imaging system via a network or any other storage device. Thus once the user has selected the appropriate compression level, the user can then save that image. With this method, the amount of data stored for each image will vary. The overall result will be saved disk space while maintaining diagnostic quality.